Attachment of roof panels to a vehicle body structure using continuous welding can offer certain advantages over traditional two-sided resistance spot welds. In additional to potential aesthetic improvements, continuous welding also allows for an enhanced design of the supporting structure beneath the roof by eliminating a need for weld access holes (conventional welding can require access to both sides of the members that are welded together), larger sections, and hydroformed tubes. If a company chooses to move toward a uniform global design vehicle, a single manufacturing process can be preferable to achieve such a uniform global design. Continuous welding, however, can require a large production-line investment or shop floor space that may not be practical for production in low-volume markets or small production plants, respectively. In such situations, it would be desirable to have a low-investment alternative joining strategy that can use standard panels without modification.
Adhesive has been used for roof panel attachment in certain aftermarket roof modifications. For example, companies specializing in modifying and customizing vehicles for specific needs often replace the standard steel roof panel. Examples include composite high roofs for disability vehicles, or composite roofs for increased cargo/recreational uses. These aftermarket modifications are typified by room temperature curing of two-component polyurethane, epoxy and acrylic adhesives. Assembly time and thus cure cycle time is usually not an issue due to the low volumes.
Adhesive has also been used for roof panel attachment when the roof panel is adhered to the frame after the vehicle has been painted and sealed. For example, the 2008 BMW® 7 series has an aluminum roof bonded to a steel body. The roof panel is positioned on the vehicle in the body shop using temporary stand-off fixtures. The gap these stands-off create allow for e-coat and paint coverage. The body color roof panel is then removed from the stand-offs in trim and final and bonded on using a low modulus two-component polyurethane adhesive. Induction heating is incorporated into the robotic handling fixture to accelerate the cure rate. Problems associated with high temperatures and differential expansion rates between the aluminum roof and steel body are avoided because the roof is attached after paint. The concept is similar to a bonded glass roof (e.g., on the 2009 Ford® Mustang). Aston Martin® uses a similar technique to BMW® for their DB9 and V8 Vantage models, although instead of an induction heating system, they use hot air impingement heating to accelerate the cure of the adhesive.
Adhesives have been used previously to augment conventional spot welds as a primary attachment means. One example is the 2004 Ford® F150, in which an adhesive was added between the 300 mm weld pitch to help with panel stiffness, durability, and reduction of noise, vibration, and harshness (NVH). The 2011 Range Rover® Evoque used a 6111-T4 aluminum roof rivet-bonded to a steel body. A heat activated, single-component, highly flexible adhesive (e.g., a Henkel Terostat 5192 adhesive) was used along with self-pierce rivets. The adhesive cured within the e-coat bake oven. Expansion problems were overcome by selective rivet placement on a 100 mm pitch and feature-lines within the roof panel to minimize permanent offset upon adhesive cure. A low modulus expanding anti-flutter mastic was also selected to minimize read-through. The self-piercing rivets employed in the Evoque required access from both sides like a conventional two-sided spot weld gun. Access holes for self-piercing rivets are actually larger than what is required for spot weld access, which can introduce water leaks.
The increased use of laser welded roofs has led to the increased use of adhesives to repair or replace them within the field. Laser welding is typically not feasible within repair shops, and other manual continuous welding processes can lead to excessive distortion when trying to weld a thin (0.7 mm) roof to a thick roof rail. Repair procedures for the Cadillac® SRX & CTS specify the use of several two-component adhesives to replace roof panels on their 2006 and later models. Ford® Europe specifies the use of a two-component epoxy adhesive for the field replacement of the laser-welded Mondeo wagon roof panel. The in-plant repair of defective laser welded roofs for the Ford® F150 specified the use of stitch welds via a single-sided welder and recommends that a two-component epoxy adhesive is spread over the roof panel to bodyside joint to fill any cracks between the welds, preventing the subsequent leakage of a flowable ditch sealer.